The GPS is a constellation of 24 earth-orbiting radio-navigation satellites whose orbits are arranged so that at least four of the satellites are visible anywhere on the Earth at any time. A GPS receiver receives signals from four or more of these satellites, and uses trilateration techniques to determine the precise position (e.g., altitude, and latitudinal and longitudinal coordinates) of the receiver on or near the Earth. Also, using the received signals, a GPS receiver computes velocity and time. Thus, GPS receivers are used to provide precise position, velocity and time information for users that include, but are not limited to, aircraft, missiles, ships, ground-based vehicles, fixed based systems, and individuals (e.g., using hand-held GPS receivers). For certain applications, differential GPS (DGPS) is used to eliminate measurement errors and improve the accuracies of the basic GPS derived position, velocity and time estimates.
A significant problem with existing GPS receivers is that they are subject to jamming and spoofing. Jamming is the intentional or unintentional denial of the signal to the user, typically through radio frequency interference (RFI). Spoofing is a surreptitious attempt (by an unfriendly party) to feed a GPS receiver false information so that the GPS receiver computes an erroneous position, velocity, and/or time. In a typical spoofing scenario, a GPS signal is received from a satellite and retransmitted with a higher power than that of the original signal in space. This indirect, deceptive GPS signal is received by a GPS receiver, which produces an inaccurate estimate of position, velocity and time, as a result. The deceptive GPS signal(s) can be derived from a retransmission of a single satellite's signal, retransmissions of a small number of satellites' signals, or retransmissions of signals from the entire visible constellation of satellites. The original, direct GPS signal can be overcome by a signal from an RF jamming device, or by the power of the indirect retransmitted signal itself. As such, spoofing has been identified by the GPS community as a viable threat. Therefore, a pressing need exists for a system that can detect and exclude false GPS signals. As described in detailed below, the present invention provides a system and method for detecting and excluding false GPS signals, which resolve the above-described spoofing problems and other related problems.